The present invention relates to a process for cooling or heating polymer in a polymerization reactor. The invention specifically relates to cooling polymerized solids in a polycondensation reactor. The invention particularly relates to cooling polyester, polyamide or polycarbonate chips in a solid-state polycondensation (SSP) reactor.
Polymer resins are molded into a variety of useful products. One such polymer resin is polyethylene terephthalate (PET) resin. It is well known that aromatic polyester resins, particularly PET, copolymers of terephthalic acid with lower proportions of isophthalic acid and polybutylene terephthalate are used in the production of beverage containers, films, fibers, packages and tire cord. U.S. Pat. No. 4,064,112 B1 discloses a solid-state polycondensation or polymerization (SSP) process for the production of PET resins.
While for fibers and films the intrinsic viscosity of the resin must generally be between 0.6 and 0.75 dl/g, higher values are necessary for molding materials such as containers and tire cord. Higher intrinsic viscosity such as greater than 0.75 dl/g can only with difficulty be obtained directly through polycondensation of molten PET, commonly called the melt phase process. The SSP process pushes polymerization to a higher degree thereby increasing the molecular weight of the polymer by the heating and removal of reaction products. The polymer with higher molecular weight has greater mechanical strength and other properties useful for production of containers, fibers and films, for example.
The SSP process starts with polymer chips that are in an amorphous state. U.S. Pat. No. 4,064,112 B1 teaches crystallizing and heating the chips in a crystallizer vessel under agitation to a density of 1.403 to 1.415 g/cm3 and a temperature ranging between 230xc2x0 and 245xc2x0 C. (446xc2x0 and 473xc2x0 F.) before entering into the SSP reactor. Otherwise the tacky chips tend to stick together.
The SSP reactor may consist of a cylindrical reactive section containing a vertical mobile bed into which the polymer chips are introduced from above and a frusto-conical portion of a dispensing section at the base for dispensing the product chips. The polycondensation reactor typically operates at temperatures between 210xc2x0. and 220xc2x0 C. (410xc2x0 and 428xc2x0 F.).
The polyester chips move through the cylindrical reactive section of the polycondensation reactor by gravity in plug-flow. However, when the chips enter the frusto-conical portion of the dispensing section at the base of the polycondensation reactor, they enter into a non-flat velocity profile which interjects a non-uniformity in the amount of time that the chips are in the polycondensation reactor. Accordingly, a chip-to-chip variation in the degree of polymerization occurs due to the variation in residence time. Moreover, at the transition from the cylinder to the cone, the chips are subjected to a consolidation pressure that may be several times the normal radial axial pressure to which the chip had previously been subjected. The chips that are in a glassy region have a strong sticking tendency. Hence, the consolidation pressure can cause lumping of chips and interruption of flow.
Various reactions occur during polycondensation of PET. The main reaction that increases the molecular weight of PET is the elimination of the ethylene glycol group:
PETxe2x80x94COOxe2x80x94CH2xe2x80x94CH2xe2x80x94OH+HOxe2x80x94CH2xe2x80x94CH2xe2x80x94OOCxe2x80x94PETxe2x86x92PETxe2x80x94COOxe2x80x94CH2xe2x80x94CH2xe2x80x94OOCxe2x80x94PET+HOxe2x80x94CH2xe2x80x94CH2xe2x80x94OH
Inert gas such as nitrogen is introduced into the crystallizer vessel and the polycondensation reactor to strip the developing polymer of volatile impurities generated by the polycondensation reaction. Impurities include ethylene glycol and acetaldehyde if PET is produced. U.S. Pat. No. 5,708,124 B1 discloses maintaining the ratio of the mass flow rate of inert gas to the mass flow rate of PET polymer solids to below 0.6 in an SSP reactor.
The conventional method used for the purification of an inert gaseous stream recycled from an SSP process includes an oxidation step for converting the organic impurities to CO2 and a drying step to eliminate the water formed in the polymerization process and the oxidation step. The oxidation step is carried out with gas containing oxygen by using an oxygen concentration of no more than in slight excess of the stoichiometric quantity with respect to the organic impurities. The oxidation step is controlled according to U.S. Pat. No. 5,612,011 B1 so that the inert gaseous stream at the outlet contains an oxygen concentration of not more than 250 ppm and preferably according to U.S. Pat. No. 5,547,652 B1 so that the inert gaseous stream at the outlet contains an oxygen concentration of not more than 10 ppm. These patents taught that a previously required deoxidation step of reducing the oxygen with hydrogen between the oxidation and drying steps was not required. Typically, the inert gaseous stream must be heated before it is recycled to the polycondensation reactor requiring additional utility cost.
It is also well known that polyamide resins, and among them particularly PA6, PA6,6, PA11, PA12 and their copolymers, find wide application both in the fiber and flexible packaging sectors, and in the manufactured articles production by blow and extrusion technology. While the resin relative viscosity for fibers is low at about 2.4 to 3.0, higher relative viscosities of 3.2 to 5.0 are needed for articles produced by blow and extrusion technologies. The relative viscosity is increased to above 3.0 by means of an SSP process operating at temperatures of between 140xc2x0 and 230xc2x0 C. (284xc2x0 and 446xc2x0 F.), depending on the polyamide types used. U.S. Pat. No. 4,460,762 B1 describes an SSP process for a polyamide and different methods to accelerate this reaction.
An SSP process for polyamide resins is also described in the article xe2x80x9cNylon 6 Polymerization in the Solid State,xe2x80x9d R. J. Gaymans et al., Journal of Applied Polymer Science, Vol. 27, 2515-2526 (1982) which points out the use of nitrogen as a heating and flushing aid. The reaction is carried out at 145xc2x0 C. (293xc2x0 F.).
It is also known that the molecular weight of polycarbonate can be increased through an SSP process.
The polymer chips exiting from an SSP reactor must be cooled to below the glass transition temperature for packaging purposes, especially to avoid heat damage to packaging containers, such as sacks and boxes. The desired packaging temperature is below 80xc2x0 C. (176xc2x0 F.) for PET chips. U.S. Pat. No. 5,817,747 B1 teaches two-stage cooling of the polymer chips after exiting the polycondensation reactor. The first cooling stage is a bed fluidized with nitrogen used for purging impurities from the SSP process after the nitrogen has been purified. The fluidizing gas entrains and separates the polymer dust from the polymer chips while cooling them to 160xc2x0 to 180xc2x0 C. (320xc2x0 to 356xc2x0 F.). The polymer dust is formed in the processing apparatuses by the action of rotating parts of an agitator in contact with the polymer chips in the crystallizer vessel and the sliding friction between the chips and the walls of the polycondensation reactor. The second cooling stage is a shell and tube or wall-type heat exchange cooler which uses water as the cooling fluid to cool the chips to between 40xc2x0 and 60xc2x0 C. (104xc2x0 and 140xc2x0 F.).
U.S. Pat. No. 5,662,870 B1 discloses a fluidized bed with two chambers for cooling polymer chips exiting the SSP reactor in a single stage. Fluidizing gas from the hotter chamber into which hot chips enter from the SSP reactor is recycled to heat the SSP reactor after it is de-dusted through a cyclone. Fluidizing gas from the cooler chamber is also de-dusted through a cyclone and recycled to the fluidizing bed. The amount of dust collected in the fluidizing gas from a fluidized bed is significant and must be removed.
JP 5-253468 A1 teaches introducing a nitrogen gas into a vessel surrounding a dispensing cone at the bottom of a reaction chamber to indirectly cool a product, solid-gas mixture in the cone without causing turbulence within the cone.
A thesis by G. Ghisolfi entitled xe2x80x9cImpianto di Postpolicondensazione di Polietilentereftalatoxe2x80x9d (1984-85) discloses an SSP reactor with nitrogen gas distributors located along the height of the reactor of which the bottom nitrogen distributor cools polyester chips to below a temperature at which oxidation can occur such as to 177xc2x0 C. (351xc2x0 F.). The chips would have to be subsequently cooled to lower temperatures to permit packaging. A presentation by A. Christel entitled xe2x80x9cAdvanced PET Bottle-to-Bottle Recyclingxe2x80x9d given at the Polyester 2000 World Congress discloses directly cooling recycled PET flakes at an outlet of a hopper of an SSP reactor with cold nitrogen.
Cooling the polymer chips after exiting the polycondensation reactor requires at least one cooling device, a gas mover such as a fan and a dust removal device. Either this equipment has to be located beneath the reactor or a pickup point and pneumatic conveying means has to be used to carry the hot chips to the top of a cooling section. Each approach involves additional equipment and building costs. Moreover, the former approach requires a taller overall SSP complex.
U.S. Pat. No. 4,255,542 B1 discloses an exothermic gas phase polymerization process in a fluidized bed reactor which is cooled by indirect cooling within the reactor. U.S. Pat. No. 3,227,527 B1 discloses a catalytic reactor vessel in which product gas permeates a truncated cone section at the base of the reactor to be cooled by quench liquid before exiting the reactor. These patents do not involve direct cooling of polymer solids in a packed bed with a gas.
Accordingly, an object of the present invention is to eliminate the additional cooling equipment required to cool the chips to a packaging temperature after exiting the SSP reactor. An additional object of the present invention is to consolidate the equipment used for cooling polymer solids and for introducing purge gas to the SSP reactor for the purging of impurities. An additional object of the present invention is to cool the polymer chips entering the dispensing section of the SSP reactor and therefore make the dispensing section a non-reactive region and to prevent the hot, tacky polymer chips from lumping when they are subjected to the consolidation pressure upon entering the frusto-conical portion of the dispensing section. An even further object of the present invention is to be able to eliminate the need for expensive dust removal equipment required with post-polycondensation reactor cooling equipment.
We have found that it is possible to cool the polymer chips from the SSP reactor temperature of 185xc2x0 to 240xc2x0 C. (365xc2x0 to 464xc2x0 F.) down to below 175xc2x0 C. (347xc2x0 C.) in a dispensing section of the polycondensation reactor. Moreover, we have found that we can cool the pellets to below 80xc2x0 C. (176xc2x0 F.) which is below the glass transition temperature for normal packaging and transport of PET chips. It was surprisingly learned that the heat transfer from gas to solids is rapid enough to allow the upper part of the dispensing section to be used for efficient cooling if particular mass ratios are used.
Accordingly, in one embodiment, the present invention relates to an apparatus for conducting a polymerization process. The apparatus comprises a reactor including a heated reactive section in which substantial polymerization of polymer solids therein occurs and a dispensing section for dispensing polymer solids from the reactor. The dispensing section defines an interior volume. A gas inlet connective with the interior volume of the dispensing section delivers gas to the dispensing section to heat or cool polymer solids traveling through the dispensing section. A gas outlet connective with the reactor proximate the dispensing section withdraws gas from the dispensing section.
In another embodiment, the present invention relates to a process for cooling polymer solids in a reactor comprising a reactive section and a dispensing section. The process comprises delivering polymer solids to the top of the reactive section. The polymer solids are polymerized as they flow downwardly in the reactive section so as to increase the molecular weight of the polymer solids. The polymer solids are dispensed from the dispensing section of the reactor. Gas is delivered to the dispensing section to flow countercurrently to the polymer solids and contact and heat or cool the polymer solids in the dispensing section. Some of the gas is withdrawn at an outlet proximate to the dispensing section.
In a further embodiment, the present invention relates to a process for adjusting the temperature of moving solids in a vessel by direct heat exchange with a gas. The process comprises delivering solids to a first section of the vessel. The solids are dispensed from a second section of the vessel. Gas is flowed through the vessel to effect direct heat exchange between the gas and the solids. A ratio of a mass flow rate of the gas multiplied by the heat capacity of the gas to a mass flow rate of solids multiplied by the heat capacity of the solids over a temperature range in the reactor is at least one.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention.